This invention relates to aluminum and more particularly, it relates to heating and melting aluminum with very high efficiency and with remarkably low melt loss or skim generation.
Aluminum is melted either continuously, that is, continuous recirculation or in static furnaces using natural gas. In natural gas fired reverberatory continuous melting furnaces, aluminum is recirculated using a molten metal pump, from the furnace, through a side bay or aluminum charging bay to a molten metal treatment bay and then back to the furnace. Aluminum metal to be melted is submerged in the charging bay. The skim or dross and other impurities resulting from the melting are removed in the melt treatment bay. Heat usually generated using natural gas is applied in the furnace.
In static furnaces, aluminum metal is charged directly to the furnace or through an open charge bay. Metal treatment may be provided using a side bay.
This method melting has the problem that it is very inefficient. That is, these furnaces operate at a 22-30% thermal efficiency because heat transfer to the melt in the furnace is effected by radiation from overhead natural gas burners to the melt. In this method of heating, large quantities of heated gases are lost as they are exhausted up the stack, creating environmental problems. This method of heating has the disadvantage that the surface temperature of the melt increases dramatically, resulting in significant skim generation and in melt loss due to oxidation of the molten aluminum. The problem is aggravated as a layer of aluminum oxide or skim forms on the surface of the melt. That is, the layer of aluminum oxide formed on the surface operates as a thermal barrier or insulator to the natural gas fire flames impinging on the surface. Aluminum oxide has a characteristically low thermal conductivity and therefore greatly inhibits heat transfer to the molten aluminum. Thus, not only is this method of heating thermal inefficient, as noted, but this method results in very high levels of melt loss due to the high surface temperature and conversion of aluminum to aluminum oxide. That is, melt loss is a significant problem encountered in this method of heating, generally averaging 2 to 5%. The high levels of skim generated in melting requires intensive molten metal treatment downstream to remove entrained skim particles.
As an alternative to reverberatory furnaces, induction melting, which can be either channel or coreless, has been used. However, coreless induction furnaces only have a thermal efficiency of about 60 to 70%, have to use a water cooled inductor surrounding the crucible and have to use a complex power supply to maintain a power factor of near unity for efficiency purposes. The power supplies are large, involve a reactor and capacitor and also must use water cooling.
Induction heating also has the problem that it stirs or agitates the melt. This constantly exposes new surface air which oxidizes the metal to form aluminum oxide. The oxides along with other impurities are mixed into the melting, resulting in serious metal quality problems. This requires intensive metal treatment with gases and/or salts downstream. This results in environmental problems from disposing of the salts. Also, it adds greatly to the expense of producing high quality metal.
Thus, it will be seen that there is a great need in the aluminum industry for a highly efficient melting system where a large portion of the heat applied is not wasted and which greatly minimizes skim or dross generation and its attendant problems of removing and treating in an environmentally responsible manner.
The present invention provides such a heating and melting system.